The synthesis and characterization of oligomers of xcex2-amino acids, named xcex2-peptides, has received considerable interest in recent years.1-4 We have been interested in the preparation of poly(xcex2-peptides) to compare these materials with poly(xcex1-peptides) and extend the homologue comparison to polymeric materials.5 Poly(xcex2-peptides) have been prepared via condensation of short peptide sequences,6-9 polymerization of xcex2-aminoacid-N-carboxyanhydrides,5,10-12 and polymerization of xcex2-lactams. 13-17 The first two methods involve tedious monomer preparations and yield only low molecular weight oligomers. However, the ring-opening of xcex2-lactams has been shown to yield high molecular weight polymers in certain cases.13-17 These polymerization reactions are not optimized in that chain length is difficult to control, and side reactions such as imide formation, racemization of chiral centers, and branching lead to heterogeneous products and low yields.14-17 
The polymerization of xcex2-lactams was first reported by Bestian,13a who prepared high molecular weight poly(xcex2-peptides) from racemic monomers bearing small alkyl side-chains. Functional side-chains, similar to those found on natural amino acids, would be more desirable since they can impart biological activity to xcex2-peptides. In this area, Muxc3x1oz-Guerra and coworkers have reported considerable studies on poly(xcex1-alkyl-xcex2-aspartates),14 taking advantage of the availability of L-aspartic acid, the only naturally occurring proteinogenic xcex2-amino acid. The xcex2-lactams of aspartic acid esters were polymerized anionically using initiators such as sodium pyrrolidone or sodium hydride (eq 1). 
Under certain conditions, racemization and the formation of imide linkages could be minimized, however chain lengths could not be controlled by monomer to initiator stoichiometry, and monomer conversions were typically seldom greater than 80%.14 The best reported control in xcex2-lactam polymerizations was obtained by Sebenda who was able to prepare narrow molecular weight distribution, low molecular weight poly(xcex2-peptides) via anionic xcex2-lactam polymerization using an N-acyl lactam activator.17 In addition to requiring xcex1,xcex1-dialkyl substituted monomers, these were not living polymerizations since proton transfer from backbone amide groups was found to deactivate the growing chains.17 Hence, branched polymers were obtained and block copolymers could not be prepared.
Recently, we have had success using metal catalysis to obtain well-defined block copolymers of xcex1-amino acids that display useful properties.18 We now report the discovery that certain metal-amido complexes can initiate the living polymerization of xcex2-lactams to give poly(xcex2-peptides) and block copoly(xcex2-peptides) with controllable chain lengths and narrow molecular weight distributions.
A series of initiators based on transition metal complexes for the polymerization of optically active beta-lactams into poly-beta-peptides and block copolymers have been developed. These initiators are substantially different in nature from all known conventional initiators used to polymerize beta-lactams and are also unique in being able to control these polymerizations so that block copolymers of beta-amino acids can be prepared. Specifically, these initiators eliminate chain transfer and chain termination side reactions from these polymerizations resulting in narrow molecular weight distributions, molecular weight control, and the ability to prepare copolymers of defined block sequence and composition. They also eliminate the formation of imide linkages in the polymers during polymerization of beta-lactams, a common detriment in conventional polymerization of these monomers. The features provided by these initiators allow the preparation of complex poly-beta-peptide biomaterials having potential applications in medicine (drug delivery, therapeutics, tissue engineering), as xe2x80x9csmartxe2x80x9d hydrogels (responsive organic materials), and in organic/inorganic biomimetic composites (artificial bone, high performance coatings).